A group of advertisers quietly filed a lawsuit in February against Google Inc., Yahoo Inc. and other Internet companies in a potentially important legal test of those companies' liability for a form of online-advertising fraud.

The plaintiffs, led by Lane's Gifts & Collectibles LLC, a Texarkana, Ark., retailer, allege that the Internet companies knowingly overcharged for advertisements they sold and conspired with each other to continue doing so. The plaintiffs are seeking to have their suit, which hasn't received widespread attention, certified as a class action.

The also named AOL, Ask, Disney, Lycos, LookSmart, and FindWhat in the suit.

The search engines have antifraud systems and sometimes issue refunds for bogus clicks. But they decline to comment in detail on the scope of the problem, exactly how they are fighting it, and any specific instances of click fraud, in part because they don't want to tip off fraudsters. That has fed some advertisers' fears that the problem is bigger than the search companies acknowledge. Estimates of click fraud run as high as 20% of all clicks on search ads.

Yahoo! has been making a strong run in the stock market for the last week, and Google is valued at 49 billion. Nobody has really challenged this issue yet. If this gets pushed it could get rather ugly quick for search stocks. Google makes 99% of their income from ads.